In the extrusion process a plastic or resin melt is formed under heat and pressure. The melt is usually formed continuously by a rotating screw in a barrel which converts solid pellets of the resin or plastic into the flowable melt. A wide variety of additions may be incorporated into the melt such as extenders or blowing agents for foam products. The nozzle of the extruder is of necessity restricted to create the required high pressure. From the nozzle the melt flows through a pipe into the die which usually includes a plenum chamber expanding to the width of the die opening. The relatively wide die opening may have adjustable die lips as in a sheet, film or board die, or it may have a series of equal circular fixed openings as in a strand die. The dies themselves are heated or cooled and may have straight, circular, or arcuate die openings. Moreover, the die openings may not be uniform across the die lips. One such die, known as a bow-tie die, is illustrated in U.S. Pat. No. 4,364,722. Other such dies are illustrated in prior U.S. Pat. Nos. 3,871,812 and 4,201,534.
One of the major problems in extrusion is to achieve complete homogeneity of the melt. This is difficult to do, particularly with regard to temperature homogeneity. Temperature, of course, affects viscosity. Even if uniform viscosity is achieved at some point in the process, it may be lost before the extrudate exits the die lips. The lower the viscosity, the faster the flow rate, and the more difficult it is to control volume throughput and the shape or profile of the product being formed. The problem is complicated by the configuration of the die. The flow path for the extrudate is not in every instance identical. For example, with fan-shape or coat hanger dies having arcuate adjustable die lips such as shown in the noted prior patents, it has been found that higher viscosity or flow is usually experienced at the center of the die with the die lips in an adjusted position. This, of course, makes the product such as a foam board, bulge in the middle. To remedy this situation, the die lips need to be readjusted to be brought closer together at the center and spread further at the lateral ends or edges. This must be done with multiple adjustments which are made even more difficult by the fact that the die may be inside a vacuum chamber. Since the die lip adjustment is preferably a vernier dial or fine adjustment, it takes longer to achieve, resulting in more out of tolerance product and removal of the die lips from an ideal setting for a given product. It would be preferable if the excess throughput at the center of the die could be corrected symmetrically by a gross adjustment without moving the die lips from an ideal setting for a given product.
In some relatively wide dies internal choker bars have been employed to achieve some form of adjustment apart from the die lips. These bars are somewhat flexible or distortable and are usually supported by a plurality of adjustment screws. By manipulation of the screws the profile of a window or opening within, the die may be controlled. Each screw has to be extended locally to restrict the opening or retracted locally to enlarge the opening. Without very expensive drives or controls, it is impossible to shift an enlargement or restriction in the window laterally of the extrudate path and substantially maintain the cross sectional area of the window. This is especially true if the enlargement or restriction is to be divided and moved symmetrically from the center to the ends or edges, or vice versa.
Also, a common form of flow control is to machine a diversion delta or control into a plenum of a die body or part, and then replace that body or part when the parameters change. Die changes in a large extrusion line are, needless to say, costly. It would accordingly be desirable to duplicate numerous types of die plenums with a simple adjustment.
It is also desirable to have a simple gross adjustment within the die, apart from the die lips, which will form an enlargement in a window within the die and enable that enlargement to be moved transversely of the flow path for the extrudate along the die lips, whether from one end to the other, or preferably, symmetrically from the center to the edges and vice versa. It would also be desirable that such gross adjustments be made quickly and conveniently from a single point or power source. It is also desirable that the ability to make such adjustments be accomplished without sharp profile or significant pressure drops within the die, which in a foam process may result in premature foaming within the die.